Posterior functionally dynamic stabilization system

ABSTRACT

A functionally dynamic stabilization unit and system for treatment of spinal instability are provided. Each unit, and collectively, the system, is configured to control flexion, extension and translation of the affected unstable vertebral area, thereby stabilizing the vertebral segments by restoring normal function. This is achieved by providing a unit and system that allow for lateral bending, axial compression, rotation, anterior segmental height adjustment, and posterior segmental height adjustment. The unit and system provide sufficient segmental stiffness, while also limiting, or controlling, the range of motion (i.e., sufficient stiffness in the neutral or active zone, while limiting or preventing motion outside of the active zone) to stabilize the vertebral segments. In use, the system mimics the natural movement of the normal spine. Furthermore, the system includes a rigid, fusion-promoting coupler configured for use in an adjacent level, or as a substitute for the functionally dynamic unit. The modularity of the system allows adjustment over time and easier revision surgery, and is configured for minimally-invasive, delivery or implantation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/952,575 filed Dec. 7, 2007 (allowed), which claims priority to U.S.Provisional Patent Application No. 60/869,342, which was filed on Dec.10, 2006, and U.S. Provisional Patent Application No. 60/914,360, whichwas filed on Apr. 27, 2007, all of which are herein incorporated byreference in their entirety.

FIELD

The present invention relates to devices and methods for treating spinalconditions, and specifically to spinal stabilization systems forcontrolling or restricting relative motion between vertebrae.

BACKGROUND

The spine includes a series of joints known as motion segment units.Each unit represents the smallest component of the spine that exhibits akinematic behavior characteristic of the entire spine. The motionsegment unit is capable of flexion, extension, lateral bending, andtranslation. The components of each motion segment unit include twoadjacent vertebrae, the corresponding apophyseal joints, anintervertebral disc, and connecting ligamentous tissue, with eachcomponent of the motion segment unit contributing to the mechanicalstability of the joint. For example, the intervertebral discs thatseparate adjacent vertebrae provide stiffness that helps to restrainrelative motion of the vertebrae in flexion, extension, axial rotation,and lateral bending.

When the components of a motion segment unit move out of position orbecome damaged due to trauma, mechanical injury or disease, severe painand further destabilizing injury to other components of the spine mayresult. In a patient with degenerative disc disease (DDD), a damageddisc may provide inadequate stiffness, which may result in excessiverelative vertebral motion when the spine is under a given load, causingpain and further damage to the disc. Depending upon the severity of thestructural changes that occur, treatment may include fusion, discectomy,and/or a laminectomy.

Current surgical treatments often involve fusion of unstable motionsegment units with removal of adjacent tissue. For numerous reasons,fusion may be an undesirable treatment option. For instance, fusionresults in a permanent, rigid fixation with irreversible loss of rangeof motion at fused vertebral levels. In addition, loss of mobility atthe fused levels causes stress to be transferred to other neighboringmotion segments, which can cause or accelerate degeneration of thosesegments. Moreover, fusion often does not alleviate some or all of thepain.

It would thus be desirable to provide a spinal stabilization system thatis sufficiently functionally dynamic to manage the load sharingcharacteristics of the treated spine. It would further be desirable toprovide a system that would allow close-to-normal motion, mimicking thephysiological response of a healthy motion segment and providing painrelief.

SUMMARY

The present disclosure provides a functionally dynamic stabilizationunit and system for treatment of spinal instability due to, for example,injury, trauma, or degenerative disc disease (DDD). Each unit, andcollectively, the system, is configured to control flexion, extension,and translation of affected vertebrae, thereby stabilizing the vertebralsegments by restoring normal function. This is achieved by providing aunit and system that allow for lateral bending, axial compression,rotation, anterior segmental height adjustment, and posterior segmentalheight adjustment. The unit and system provide sufficient segmentalstiffness, while also controlling the range of motion to stabilize thevertebral segments. In use, the system mimics the natural movement ofthe normal spine. Furthermore, the system is configured to allowadjustment over time, revision surgery (e.g., fusion), and percutaneousimplantation.

In accordance with one exemplary embodiment, a functionally dynamicspinal stabilization system is provided. The system may comprise aflexible coupler and can include a cylindrical body portion includingone or more slots in the wall of the cylindrical body. The system canfurther include a pair of gripping arms for attachment to bone anchors,the arms being located at opposed ends of the coupler. The flexiblecoupler may also include an internal range-of-motion limiting mechanismconfigured to limit motion of the flexible coupler in bending,compression, and tension. The system can further comprise a pair of boneanchors configured to cooperate with the gripping arms for attachment tobone tissue.

In accordance with another exemplary embodiment, the system furtherincludes a rigid coupler having a pair of gripping arms for attachmentto bone anchors. Like the flexible coupler, the arms can be located atopposed ends of the coupler. However, unlike the flexible coupler, thiscoupler does not allow extension or compression. Rather, the couplerpromotes fusion by preventing motion at this segment.

Also provided is a method of treating a spine. The method can compriseattaching a first bone anchor to a vertebra and attaching a second boneanchor to an adjacent vertebrae. A flexible coupler may then be attachedto the first and second bone anchors. The flexible coupler can include acylindrical body portion having one or more slots in the wall of thecylindrical body and an internal range-of-motion limiting mechanismconfigured to limit motion of the flexible coupler in bending,compression, and tension.

Also provided is a method of percutaneous implantation of the systemthat minimizes tissue damage and eases insertion, as well as aninstrument set for performing this method. The method can includeproducing at least one incision over at least two adjacent vertebrae tobe treated and positioning at least two wires such that each wireseparately contacts a pedicle of one the at least two vertebrae. A screwmay be secured to each vertebrae, and the distance between the screwsinserted into two adjacent vertebrae is measured. A flexible coupler tobe attached to the screws is selected, and the length of the flexiblecoupler is adjusted based on the distance measured.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thedisclosure and together with the description, serve to explain theprinciples of the disclosure.

Additional objects and advantages of the disclosure will be set forth inpart in the description which follows or may be learned by practice ofthe disclosure. The objects and advantages of the disclosure will berealized and attained by means of the elements and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side perspective view of an implanted functionallydynamic stabilization system.

FIG. 2 illustrates a top view of the implanted functionally dynamicstabilization system of FIG. 1, including two stabilization units onopposite sides of the spine.

FIG. 3 illustrates a posterior view of the system of FIGS. 1-2.

FIG. 4A illustrates a perspective view of one stabilization unit of thesystem of FIGS. 1-3.

FIG. 48 illustrates a side view of a portion of a flexible coupler usedin the stabilization unit of FIG. 4A.

FIG. 4C illustrates a top view of he flexible coupler of FIG. 4B.

FIG. 5A illustrates a cross-sectional view of he unit of FIG. 4A.

FIG. 5B illustrates an exploded view of a portion of the stabilizationunit of FIG. 4A.

FIG. 6 illustrates an exploded view of the flexible coupler of FIGS.4B-4C.

FIG. 7A illustrates a perspective view of a portion of the flexiblecoupler of FIGS. 4B and 4C.

FIG. 7B illustrates a perspective view of a section of the portion ofthe flexible coupler of FIG. 7A.

FIG. 8A illustrates a cross-sectional view of the flexible coupler ofFIG. 4B in a resting state.

FIG. 8B illustrates a cross-sectional view of the flexible coupler ofFIG. 4B in a fully expanded state.

FIG. 8C illustrates a cross-sectional view of the flexible coupler ofFIG. 4B in a fully compressed state.

FIG. 8D illustrates an enlarged view of a portion of the flexiblecoupler of FIG. 8A in a resting state.

FIG. 9A illustrates a perspective view of another embodiment of animplanted functionally dynamic stabilization system.

FIG. 9B illustrates an enlarged view of the implanted system of FIG. 9A.

FIG. 10 illustrates a side view of a portion of the system of FIGS.9A-98.

FIG. 11A illustrates a perspective view of a rigid coupler hat may beused with the stabilization systems of the present disclosure.

FIG. 11B illustrates a cross-sectional view of the rigid coupler of FIG.11A taken along line A-A.

FIG. 11C illustrates a side cross-sectional view of an alternativeembodiment of rigid coupler that may be used with the stabilizationsystems of the present disclosure.

FIG. 12 illustrates a perspective view of a modular, multi-segmentalstabilization system, according to another embodiment of the disclosure.

FIG. 13 illustrates a perspective view of a wire template and K-wiresused to facilitate implantation of the spinal stabilization systems ofthe present disclosure.

FIG. 14A illustrates a perspective view of a set of extension rods usedto facilitate implantation of bone anchors using the methods of thepresent disclosure.

FIG. 14B illustrates a partial cutaway view of one of the extension rodsof FIG. 14A connected to a bone anchor.

FIG. 15 illustrates a perspective view of a caliper.

FIG. 16 illustrates a perspective view of an alternative of extensionrods according to the present disclosure.

FIG. 17 illustrates a perspective view of an instrument for adjustingthe length of a flexible coupler.

FIG. 18A illustrates a perspective view of a nut that ay be used tosecure stabilization units of the present disclosure.

FIG. 18B illustrates a partial cutaway view of the nut of FIG. 18Bcoupled to the insertion tool of FIG. 19.

FIG. 19 illustrates a perspective view of an insertion tool.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a functionally dynamic stabilizationunit and a system incorporating functionally dynamic stabilization unitsfor treatment of spinal instability. The present disclosure furtherprovides minimally-invasive methods for implanting spinal stabilizationsystems, as well as instruments that will facilitate these methods.

The unit, system, and methods of the present disclosure may be used totreat spinal pathologies caused by, for example, injury, trauma, ordegenerative disc disease (DDD). The stabilization unit and systemscomprising such units are configured to control flexion, extension andtranslation of an affected unstable vertebral area, thereby stabilizingvertebral segments and restoring normal function. This is achieved byproviding a unit and system that allow for lateral bending, axialcompression, rotation, anterior segmental height adjustment, andposterior segmental height adjustment on the spine. The unit and systemprovide sufficient segmental stiffness within a patient's neutral oractive zone, while also limiting or controlling range of motion outsidea desired zone. In use, the system mimics the natural movement of thenormal spine. Furthermore, the system is configured to allow adjustmentover time, revision surgery, and percutaneous delivery or implantation.

Turning now to the drawings, FIG. 1 shows an embodiment of afunctionally dynamic stabilization system 8, implanted between adjacentvertebrae 2, 4. FIG. 2 illustrates a top view of an implantedfunctionally dynamic stabilization system, and FIG. 3 illustrates aposterior view of the system 8 of FIGS. 1-2. As shown, the system 8 caninclude one or more flexible stabilization units 10 that can beimplanted on a posterior portion of the spine to stabilize affectedvertebrae 2, 4.

As shown in FIG. 4A, each functionally dynamic stabilization unit 10 maycomprise a flexible coupler 20 connected to at least one bone anchor 50,such as a pedicle screw or bone screw. The coupler 20 may comprise aflexible body 22 including slots 24 and openings 26. As shown in FIGS.48-4C, the flexible body 22 may include, at one end, a gripping arm 30having an opening 32 for insertion of a bone anchor 50, and at anopposite end a second gripping arm 40, also having an opening 33 forreceiving a bone anchor 50. The gripping arms 30, 40 may be integrallyformed with the body 22 or may be detachably connected to the body 22.For example, one end of the gripping arm 40 may be threaded forconnection to the flexible body 22 via, for example, a sleeve 90 in theflexible body 22, as shown in FIG. 6.

Each gripping arm 30, 40 of the coupler 20 can include, on one side, aconcavely-shaped cavity 34, 44 configured to seat against asemi-spherical ball bearing 60, shown in FIGS. 5A-5B, and 6. The ballbearing 60 can have a through-hole, allowing it to fit over the boneanchor 50. In one embodiment, the bone anchor 50 may have an elongate,threaded shaft 52 extending into a flange 56 that connects to a headportion 54 upon which the ball bearing 60 may be placed. The flange 56may further include serrations 57 to facilitate anchorage to bone tissueand reduce loosening of the anchor 50 over time. The bone anchor 50 maybe, for example, a pedicle screw. Preferably, the bone anchor 50 can becannulated to enable the unit 10 or system 8 to be percutaneouslydelivered. The concavely-shaped cavities 34, 44 allow the gripping armsto slide or rotate with respect to the bearing 60, thereby enabling thegripping arms 30, 40 to move relative to the bone anchor 50. Otherappropriate structures may be used to connect the flexible body 22 tothe bone anchors 50 while permitting relative movement between the two.

As further shown in FIGS. 5A, 5B and 10, a washer 70 may be placed ontothe screw 50 and against the flange 56 or nut 80. The washer 70 can beconfigured and shaped to lie against the ball bearing 60. An assembledfunctionally dynamic stabilization unit 10 would further include a nut80 screwed onto the head portion 54 of the screw 50 to secure thecomponents to one another, as illustrated in FIGS. 4A and 5A.

Each functionally dynamic stabilization unit 10 is configured to allow arange of motion or displacement of between 1.5 and 3.0 mm, wheredisplacement may be measured from the center of a first pedicle screwconnected to a first gripping arm 30 to the center of a second pediclescrew connected to the second gripping arm 40. This displacement orrange of motion may be achieved, for example, through rotation,extension, or translation.

FIG. 6 illustrates an exploded view of the flexible coupler of FIGS.4A-4C. As shown, in some embodiments, one of the gripping arms 40 may beremovably attached to the coupler 20. In one embodiment, the coupler 20can include a threaded opening 28 for securing the second gripping arm40, and other components, to the coupler 20. Within the flexible coupler20, there may be a sleeve 90 having an opening 92 at one end andincluding a threaded rim 94 around the opening 92 for threadablyconnecting to the coupler body 22. The sleeve 90 can be configured toreside within the coupler body 22 and to receive and cooperate with apin 100. The pin 100 can comprise an elongate body 102 with a threadedend, the body 102 extending into a semispherical head region 104 andincluding a skirt or shoulder region 106. Collectively, the sleeve 90and pin 100 form an extension and compression stop within the couplerbody 22, functioning to limit range of motion of the flexible coupler 20to the patient's neutral or active zone.

The rim 92 of the sleeve 90 may be threaded to engage the threaded end46 of the detachable second gripping arm 40. The overall length of thecoupler 20 may be adjusted by varying the amount of threading of thesecond gripping arm 40 into the sleeve 90 (i.e., varying the number ofrotations of the arm 40 into the sleeve 90). As shown, the threaded end46 of the detachable second gripping arm 40 may extend into a pluralityof compressible finger projections 43, each projection 43 terminating ata flanged lip 47. The flanged lip 47 serves as a locking mechanism,preventing the second gripping arm 40 from being unscrewed from thesleeve 90 after assembly. The threaded end 46 may also include a well 48for receiving an elastomeric plug 110, as shown in FIG. 8C. Theelastomeric plug 110 may be formed of a, soft, compliant plasticmaterial such as, for example, silicone, polyethylene, orpolyethyletherketone (PEEK). As the second detachable gripping arm 40 isthreaded onto the sleeve 90, the plug 110 interacts with the threadedopening 92, reducing the slack or play between the arm 40 and the sleeve90. Other suitable structures that permit adjustment of the length ofthe flexible body while providing control of the amount of compressionand extension of the flexible body may also be used. For example, agripping arm can be attached at a friction fit, a telescopingconnection, or using a ratchet mechanism.

As shown in detail in FIGS. 7A and 7B, in one exemplary embodiment, thecoupler body 22 may include a cylindrical body comprised of a series ofcoil units 22A. The series of coil units 22, when connected to oneanother to form a stepwise series of slots 24, whereby each slot 24terminates at an opening 26 of the flexible body 22. In someembodiments, the series of coil units 22A can be formed from a singlepiece of material such that the units 22A are integrally connected withone another. For example, in one embodiment, the coil units 22A can beetched or cut from a single, tubular piece of material. In otherembodiments, one or more coil units 22A can be formed individually andstacked upon one another. The stacked coil units 22A can be connected toone another, for example, by welding or through mechanical connections.

It is contemplated that the coupler body 22 may vary in degree ofstiffness based on the height, width, distance or angle between twoadjacent slots 24 and the number of units 22A forming the coupler body22. Further, one or more units 22A may be formed from differentmaterials so as to vary the mechanical properties of the body 22. Inaddition, the dimensions of the units 22A, slots 24, and openings 26 canbe varied within a single body 22.

FIGS. 8A-8D show an embodiment of the fully assembled flexible coupler20 in a resting state (FIGS. 8A and 8D), fully-expanded or distractedstate (FIG. 8B), and a fully compressed state (FIG. 8C). In the restingstate, shown in FIG. 8A and an expanded view in FIG. 8D, the pin 100 andsleeve 90 are not engaged (i.e., free of resistive forces orencumbrances). In the fully-expanded or distracted state (FIG. 8B), thepin head 104, having a dimension that is larger than the width of thenarrowed opening 98, abuts the narrowed opening 98 of the sleeve 90,preventing the flexible coupler body 22 from over expanding. In thefully-compressed state (FIG. 8C), the end of the sleeve 90 with thenarrowed opening 98 abuts the inner edge of the first gripping arm 30,as shown. The cooperation of the sleeve 90 and pin 100 inside thecoupler body 22 provides a distraction-compression stopping mechanism tocontrol or limit the range of motion that can be offered, preventing notonly injury or damage to the affected vertebral segments but also to thefunctionally dynamic stabilization unit itself. Other types ofcooperating elements, such as, for example, a telescoping element orinternal piston, may be sued to control or limit the range of motion ofthe coupler body 22.

As previously mentioned, the functionally dynamic stabilization unit 10may be used alone to stabilize a pair of vertebral segments. Further, ifdesired, more than one unit 10 may be used in combination to form amulti-level, functionally dynamic stabilization system 12, as shown inFIGS. 9A and 9B. The multi-level, functionally dynamic stabilizationsystem 12 may include two or more of the units 10 connected to oneanother.

FIG. 10 illustrates a side view of the system shown in FIGS. 9A-9B. Asshown, the system 12 includes a pair of flexible couplers 20 connectedin series. The couplers 20 are positioned such that the first grippingarm 30 of each coupler 20 is placed around one ball bearing 60, with abone anchor 50 and nut 80 securing the combination together. It isunderstood that more than two couplers 20 may be connected in thismanner, and either the first 30 or second 40 gripping arm of any singlecoupler may be combined with the first 30 or second 40 gripping arm ofanother coupler 20 on a bone anchor 50. Any number of couplers 20 may beimplanted either along one side, or on both sides, of a patient's spine.Further, the units 10 may have differing mechanical properties accordingto the patient's pathology and anatomy.

In some embodiments, the stabilization systems of the present disclosurecan allow fusion of one or more vertebral motion segments, along withfunctionally dynamic stabilization of other motion segments. To thisend, the stabilization system may include a rigid, fusion-promotingcoupler 101, such as the one shown in FIG. 11A. The rigid coupler 101can be configured for use with the bone anchors 50, ball bearings 60,and washers 70 described previously. As illustrated, the rigid coupler101 comprises two components 122, 124, each of which extends to agripping arm 130, 140, respectively, in a manner similar to that in theflexible coupler 20 previously described. Each of the arms 130,140includes an opening 132 for attachment to a bone anchor 50, in a mannersimilar to that described with respect to the flexible coupler 20.

As further shown in FIG. 11B, the two components 122,124 may be attachedto one another to allow adjustment of the length of the rigid coupler101. For example, the components 122,124 can include threaded surfaces,and the length of the rigid coupler 101 can be adjusted by twisting onecomponent 122 with respect to the other component 124, much like themanner previously described for adjusting the length of the flexiblecoupler 20. Each of the gripping arms 130,140 can also include, on anunderside, a concave cavity 134,144, respectively, configured to seatagainst a semi-spherical ball bearing 60. Hence, the implantation of therigid coupler 101 to the bone anchors 50 is similar to that for theflexible coupler 20, as previously described.

As shown in FIG. 11C, an alternative embodiment of a rigid,fusion-promoting coupler 201 may be provided. The rigid,fusion-promoting coupler 201 is similar to rigid coupler 101 except thatit may not utilize threaded surfaces of components for adjusting alength of the coupler 201. The rigid coupler 201 can be configured foruse with the bone anchors 50, ball bearings 60, and washers 70 describedpreviously. As illustrated, the rigid coupler 201 comprises twocomponents 222, 224, each of which extends to a gripping arm 230, 240,respectively, in a manner similar to that in the flexible coupler 20previously described. Each of the arms 230, 240 includes an opening (notshown) for attachment to a bone anchor 50, in a manner similar to thatdescribed with respect to the flexible coupler 20. Each of the grippingarms 230, 240 can also include, on an underside, a concave cavity 234,244, respectively, configured to seat against a semi-spherical ballbearing 60.

The first component 222 and the second component 224 may be movablerelative to one another to facilitate adjustment of the length of thecoupler 201. Instead of threaded surfaces, the component 222 may includea cavity 226 configured to receive a fastening element 230 to secure thefirst component 222 relative to the second component 224. Because thefirst and second components do not include threaded surfaces, they maybe moved relative to one another by sliding the components rather thantwisting. Such an embodiment permits the surgeon to adjust the length ofthe rigid coupler 201 in situ as necessary.

The fastening element 230 may be any suitable fastening element such asa screw or a nut. For example, the fastening element 230 may comprise abreak-away nut having a first portion configured to fixingly engage theportion 226 of component 222 to fix the position of the first component222 relative to the second component and a second portion configured toengage an insertion tool for tightening of the first portion to therigid coupler. The second portion of the break-away nut may be abreak-away portion that has a thinner wall or area of loweryield-strength material, and is configured to break when a sufficienttorque is applied (i.e., when the nut 230 has been sufficientlytightened). An internal surface of cavity 226 and an external surface ofthe fastening element 230 may be provided with threads to facilitateengagement of the cavity 226 with the fastening element 230.

As noted, the stabilization system may include both functionallydynamic, flexible couplers 20 and rigid couplers 101, thereby providinga modular system that allows the combination of motion preservation andfusion at discrete segments of the patient's spine. By permittinginterchangeability of the rigid couple 101 and a flexible coupler 20, inthe system, the surgeon will have greater flexibility to address thespecific needs of the patient. Therefore, one spinal segment may havefunctionally dynamic stabilization (i.e., non-fusion while an adjacentsegment may have rigid, segmental fixation (i.e., fusion).

FIG. 12 illustrates a multi-segmental system 12 comprising threediscreet stabilization units 10 a, 10 b, 10 c utilizing flexiblecouplers 20 a, 20 b and a rigid coupler 101. The flexible couplers 20 a,20 b of units 10 a and 10 c increase the segmental stiffness of theaffected motion segment and restrict the range of motion in flexion,extension, lateral bending and rotation, while preserving motion. Byselecting an appropriately-sized coupler 20 a, 20 b, the posteriorsegmental height can be adjusted as well. In addition, the rigid,fusion-promoting coupler 101 of unit 10 b provides rigid, segmentalfixation, thereby promoting fusion, while utilizing the same type ofbone anchors 50 and instruments.

The modular system 12 provides a number of advantages. For example,initially, an implanted system may include only functionally dynamic,flexible couplers 20 connected to vertebra with bone anchors 50, asdescribed above. However, subsequently, due to progression in disease,unabated pain, other symptoms, or other changes in a patient'scondition, it may be desirable to fuse one or more previously-treatedlevels. Therefore, in subsequent surgeries, a surgeon can simply replacea previously-implanted flexible coupler with a rigid coupler 101, whilelikely using the same bone anchors.

As noted previously, the units and systems of the present disclosure canbe implanted using a minimally-invasive, muscle-sparing approach. Suchapproaches can include percutaneous methods or a series of smallincisions that minimize tissue damage.

FIGS. 13-19 illustrate exemplary embodiments of insertion instrumentsthat may be provided separately or as a set along with the system. Inone exemplary method of the present system, a series of K-wires 200 areinserted into the pedicles of the patient's spine. The K-wires 200 maybe inserted through a series of small incisions in the patient's back.Further, as shown in FIG. 13, a wire template 202 may be provided toassist the surgeon in placement of the incisions and K-wires 200. Thewire template 202 may include predetermined openings 204 that align withthe pedicles of the patient's spine, as illustrated. The openings 204may be bilaterally located in line with both pedicles of vertebrae to betreated. The template may be provided in various sizes to accommodatepatients having variations in pedicle spacing.

After insertion of the K-wires 200, the cannulated bone anchors 50 maybe passed over the K-wires 200, and using a series of extension rods 220a, 220 b, 220 c, shown in FIG. 14A, the bone anchors can be implantedwithin selected vertebra. As shown in FIG. 14B, the extension rods canattach to the head portions 54 of the bone anchors 50 to allowmanipulation of the anchors 50. In addition, a dilatation sleeve (notshown) can be provided, and the extension rods can be passed through thedilation sleeve to access the implantation site. After or duringimplantation of the bone anchors 50, the extension rods 220 can be usedto manipulate the anchors 50 and the attached vertebrae to ascertain thefull range of motion in a static condition and with an applied load.Such information may be useful to the surgeon to predict the possiblerange of corrective motion desirable for that spine segment.

A caliper 240, as illustrated in FIG. 15, may also be provided with theinstrument set. The caliper 240 can comprise a pair of pivoting arms242, 244, each arm extending to a finger engaging opening 246, 248,respectively, and terminating at an opposite end into a gripping end250, 252, respectively. The pivoting arms 242, 244 can be connected viaa leaf spring 254. As shown, the ends of the arms 242, 244 areconfigured to provide a reading or measurement of the distance between apair of adjacent bone anchors 50 using the indicia markings 258 on abackboard 256. The gripping ends 250, 252 can be configured to hold aportion of the ball bearing 60 of each bone anchor 50. This enables thecaliper 240 to function even when the bone anchors 50 are situated in anonparallel or unique angle relative to one another,

FIG. 16 illustrates various rod extensions 260 that are configured toconnect to other components of the anchor, such as the ball bearing 60,washer 70, or nut 80. Each of these rod extensions 260 enablesminimally-invasive or percutaneous manipulation of the respectivecomponent.

Once the bone anchors 50 are in place and the distance between a pair ofadjacent bone anchors 50 has been determined, a surgeon may then selecta suitably-sized functionally dynamic, flexible coupler 20 or a rigid,fusion-promoting coupler 101 for placement between the anchors 50. Acoupler length adjuster 270, similar to the one shown in FIG. 17, may beprovided to ensure that the coupler length is correct prior toinsertion. As illustrated, the length adjuster 270 may include a body272 having a pair of grips 271, between which a coupler 20,101 can beheld. The pair of grips 271 form the insertion area 274 for the coupler.Within the body 272 is a spring-loaded mechanism that exerts biasedforce against one of the grips 271. The spring-loaded mechanism may becontrolled by turning a knob 280, thereby twisting the coupler 20,101,and consequently adjusting its length. The body 272 may further includea window 278 within which there appear indicia 276 indicating the lengthof the coupler. Although a flexible coupler 20 is illustrated, it isunderstood that the length adjuster 270 is also applicable for use witha rigid coupler 101.

The appropriately-sized coupler 20,101 is then slid down the K-wires 200and onto the ball bearings 60 of the bone anchors 50. Subsequently, nuts80 may be used to secure the coupler 20, 101 in place. In someembodiments, the nuts 80 may have features that prevent over- or undertightening. For example, FIG. 18A illustrates an exemplary embodiment ofa suitable nut 180 having a break-away portion 182, connecting ananchor-engaging lower portion 186 to an upper portion 184. Thebreak-away portion 182, having a thinner wall or area of loweryield-strength material, is configured to break when a sufficient torqueis applied (i.e., when the nut 180 has been sufficiently tightened).

The nut 180 can be inserted through the minimally-invasive approach usedto implant the bone anchors 50 and couplers 20, 101. For example, FIG.19 shows an exemplary insertion tool 290 useful for insertion of the nut180. The insertion tool 290 comprises an elongate body 292 extendingfrom a handle portion 294 to a nut coupling end 296 at an opposite end.The coupling end 296 may be configured to securely attach to the nut atthe upper portion 184, as shown in FIG. 18B, and the elongate body 292,with a nut coupled thereto, can be inserted into a previously definedaccess site to secure the nut 180 to a bone anchor 50. With sufficienttightening, the nut 180 will break at break-away portion 182, leavingthe lower portion 186 on a bone anchor and allowing the upper portion184 to be withdrawn.

The surgeon may elect to repeat this process at an adjacent level untilall the affected levels of the patient's spine have been treated. Theentire process may be done percutaneously and/or with minimal disruptionto the surrounding tissue.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of thedisclosure provided herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the disclosure being indicated by the following claims.

What is claimed is:
 1. A spinal stabilization unit, comprising: aflexible coupler having a body, a pair of arms, the arms being locatedat opposed ends of the coupler, and a range-of-motion limiting mechanismconfigured to control an amount of bending, an amount of compression,and an amount of extension of the coupler; and an anchoring systemincluding a plurality of bone anchors configured to cooperate with thearms of the flexible coupler to attach the coupler to bone; wherein therange-of-motion limiting mechanism comprises a sleeve extendinginternally from a first end of the coupler towards a second end of thecoupler and having a narrowed distal opening, and an elongated bodyextending internally from the second end of the coupler towards thefirst end of the coupler and having an enlarged end disposed within thesleeve and dimensioned such that the enlarged end abuts the wall of thenarrowed opening when the coupler is elongated or bent, and the sleeveabuts the second end of the coupler when the coupler is compressed. 2.The unit of claim 1, wherein at least one of the arms is connected tothe body of the flexible coupler at a threaded connection.
 3. The unitof claim 1, wherein the body is flexible.
 4. The unit of claim 1,wherein the body is extendable and compressible along a longitudinalaxis of the body.
 5. The unit of claim 1, wherein the body is bendablealong its long axis.
 6. The unit of claim 5, wherein each arm includes aconcave portion having an opening, and the anchoring system furtherincludes a semispherical bell bearing for each bone anchor.
 7. The unitof claim 6, wherein the flexible coupler is movable relative to theplurality of bone anchors.
 8. The unit of claim 1, wherein the body iscylindrical and includes a plurality of elements forming slots withinthe body.
 9. The unit of claim 1, wherein a length of the flexiblecoupler is adjustable.
 10. The unit of claim 9, wherein one of the armsis attached to the coupler at a threaded connection, and the length ofthe coupler can be adjusted by rotation of the arm with respect to thethreaded connection.
 11. The unit of claim 1, wherein the movement ofthe elongate body relative to the sleeve in a first direction defines arange of extension of the coupler, and movement of the elongate bodyrelative to the sleeve in a second direction, opposite to the firstdirection, defines a range of compression of the coupler.
 12. The unitof claim 1, wherein the bone anchoring system further includes at leastone ball bearing and at least one nut.
 13. The unit of claim 1, whereinthe flexible coupler is configured to be movable relative to theanchoring system.
 14. The unit of claim 1, wherein the anchoring systemfurther includes a plurality of nuts, each nut having a narrowedbreak-away portion configured to break when sufficient torque is appliedto the nut.
 15. The unit of claim 1, wherein the range-of-motionlimiting mechanism is located within the body.
 16. A method ofimplanting a spinal stabilization unit, comprising: providing at leastone incision over at least two adjacent vertebrae to be treated;positioning at least two wires into pedicles such that each wireseparately contacts a pedicle of one of the at least two vertebrae;securing a screw to each of the first and second adjacent vertebrae tobe treated; adjusting a length of a flexible coupler to fit between twoof the screws; and attaching the flexible coupler to the two screws ofthe first and second adjacent vertebrae.
 17. The method of claim 16,further including measuring the distance between the screws secured tothe two adjacent vertebrae.
 18. The method of claim 16, whereinadjusting the length of the flexible coupler includes rotating an arm offlexible coupler to adjust an amount the arm extends from a body portionof the flexible coupler.
 19. The method of claim 16, further comprising:providing at least one incision over a third vertebra to be treated;positioning at least one wire into a pedicle of the third vertebra suchthat the wire separately contacts the pedicle of the third vertebra;securing a screw to the third vertebra to be treated; and attaching arigid coupler to the screw of the third vertebra, and to one of thescrews of the first and second vertebrae.
 20. The method of claim 19,further including measuring the distance between the screw of the thirdvertebra and to one of the screws of the first and second vertebrae. 21.The method of claim 20, further adjusting a length of the rigid couplerto fit between the screw of the third vertebra and to one of the screwsof the first and second vertebrae.
 22. The method of claim 21, whereinadjusting the length of the rigid coupler includes rotating an arm ofthe rigid coupler to adjust an amount the arm extends from the rigidcoupler.